This step occurs usually right before your book is published at galley stage.
Galleys are uncorrected proofs of your novel that your publisher sends you. Ask your publisher for more than one copy. As soon as your receive it, immediately send that extra copy to Publishers Weekly, the major trade magazine for publishers, librarians, booksellers, and newsletters. PW's galley reviews offer maximum exposure and a head-start for promotions. A warning: Publishers Weekly will only review novels in galley. I didn't know this fact. As a result, PW rejected my novel for review, and my quest for reviews was immediately off to a slow start.
When your novel is published, request about 100 copies for yourself. You may have to pay for these copies at a deep-discounted author's rate. Many publishers, however, will give their authors a certain number of freebie copies as an amenity.
Compile a list of newspaper, magazine, and online book reviewers contact information. Do an online search, and you'll find plenty of information. Bookmarket.com is a prime site for this type of information.
Send letters of inquiry to reviewers, preferably by email. Start with your local media reviewers. As a favorite son or daughter, reviews from these venues are easiest. Next, send letters to reviewers at your favorite newspapers, magazines, or websites. After that, send inquiries to the rest. This is a time consuming process.
Keep your inquiries concise, and do offer to send a free copy of your novel. And, of course, include a brief description of your novel with the intent of tantalizing the reviewer.
When reviewers respond and ask for a copy of your book, this is where your campaign will start to get expensive. Prepare a budget for postage and envelopes.